Add initial multithreading support and example

This is the basic support for doing i/o in a separate worker thread.
It is still not threads safe but a start.
Now we need to protect all variables such as outqueue, waitpdu
and friends.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>

lib/libiscsi.syms.in

@@ -41,6 +41,8 @@ iscsi_modesense10_sync
 iscsi_modesense10_task
 iscsi_modesense6_sync
 iscsi_modesense6_task
+iscsi_mt_service_thread_start
+iscsi_mt_service_thread_stop
 iscsi_nop_out_async
 iscsi_orwrite_iov_sync
 iscsi_orwrite_iov_task